{% (this is a block) %}
${ (this is an expression) }
$this_is_a_variable
${ this_is_also_a_variable }
${ variable | this_is_a_filter("and", "these are", arguments) }
$variable.these.are.attributes
this == is + an ~ "expression"
{# this is a comment, and is removed when the template is built #}

0
down vote
I figured it out, I was able to print the term ID of the category field using:

{{ node.field_category.entity.tid.value }} as I realised I was looking in the wrong direction. Drupal 8 use entity reference only so I needed to call the TID from the referenced entity which of course is a taxonomy term in my category field.